Premature babies require intensive nursing and medical care over the first few critical days of life. The chances of survival for these premature babies is dramatically increased if care is taken to keep them warm.
Traditionally, a warm environment has been provided in which the premature infant may subsist in the form of an incubator. The air inside the incubator is heated to a preselected temperature and circulated by a fan. The air temperature is either selected according to the baby's weight and post-natal age, or by using a servo control system to maintain the temperature of a skin site at an appropriate value.
Another type of neonatal bed employed in neonatal intensive care units is the radiant warmer. In a radiant warmer a radiative heat source is located within an enclosure. Radiated heat from the source maintains the skin temperature of the premature infant at a predetermined level, again by either correlating the desired air temperature to the baby's weight and post-natal age, or by using a servo control system to maintain the temperature of a skin site at an appropriate value.
A problem facing manufacturers of incubators and radiant warmers is the lack of a suitable infant thermal simulator for use in testing and evaluation of incubators and radiant warmers which accurately models the thermal characteristics of a premature infant.
A number of devices have heretofore been employed to attempt to simulate the thermal properties of an infant in order that the testing of incubators and radiant warmers could be performed. Some of these devices include a water-filled copper shell, a water-filled child's doll which has an internal heater, a segmented aluminum mannikin filled with water, an internally heated hollow copper ellipsoid filled with water, a mean hemispherical radiant temperature thermopile, a black anesthesia bag filled with water and a hollow copper sphere filled with water.
All of the above prior art devices have been deficient in modeling the thermal characteristics of a premature infant primarily because these devices do not account for heat losses due to water loss as a result of evaporation ("insensible water loss") encountered by a premature infant. The premature infant loses heat by a combination of conduction, convection, radiation and evaporation. Therefore, any mechanical model which simulated only conduction, convection, and radiation would not properly thermally model the thermal characteristics of a premature infant.
Conduction of heat takes place from the skin of an infant to the mattress of the bed. Convective loss occurs by normal respiratory expiration and by convective heat transfer at the skin surface. Heat transfer by radiation is a function of temperature gradients and the effective areas for exchange between the infant's skin and the surrounding surfaces. Lastly, evaporative heat loss is primarily influenced by the air velocity and the relative humidity in the convective incubator and the dissipation through absorption of infrared energy on the surface of the skin with radiant warmers. Each gram of water evaporated consumes 0.58 kcal of heat. For an incubator these water losses contribute to the overall heat losses of the infant. For a radiant warmer the heat necessary to support the evaporation process is provided by the infrared heating source with the management problem being that of maintaining the appropriate water balance through increased fluid intake or shielding the infant to reduce water losses. The natural diffusion of water through the skin and its subsequent evaporation can account for approximately 50% of the heat loss in a premature infant. Accordingly, any infant thermal simulator which does not account for these evaporative losses could therefore be in error by as much as 50%.
A mechanical infant simulator must therefore exhibit certain physical characteristics; at the same time, however, the simulator must be as anatomically correct as possible. In order to properly simulate the above heat transfer modes, the simulator must have a mass, surface area, metabolic heat production, insensible water loss and heat storage capacity corresponding to the age (and size) of the infant simulated.
Anne E. Wheldon and Michael M. Donnelly, in respective papers entitled "Energy Balance Of A Newborn Baby: Use Of A Mannequin To Estimate Radiant And Convective Heat Loss," published in 1982 by the Institute of Physics, and "Essential Background For The Design Of A Neonatal Bed," published in 1988 by the Perinatal Research Institute, Cincinnati, Ohio, proposed the use of a combination of cylinders and spheres with which to model the thermal characteristics of a premature infant. In the Wheldon paper, the simulator had a spherical head, a cylindrical trunk with two closed ends and four cylindrical limbs, each with one closed end. The head was attached to the trunk by a short polystyrene "neck". The open ends of the limbs were fitted with polystyrene plugs which were opposed to polystyrene disks on the trunk. The spherical head was fabricated of a thin copper ballcock; 20-gauge aluminum tubing was utilized for the curved surfaces of the limbs and trunks and 16-gauge aluminum sheet for the tube ends. The simulator was designed to represent an average full term newborn of birth weight 3.3 kg and surface area 0.23 m.sup.2. The diameters and lengths of the limbs and trunk were selected to give the correct regional surface areas. The simulator was heated using resistance wire.
In the Donnelly paper, it was proposed that separate simulators be fabricated to simulate infants less than 1,000 grams and infants greater than 2,500 grams due to the significantly different thermal properties of each. Heat loss for infants with birth weights greater than 2500 grams would not require an independent insensible water loss simulation to account for this mode of heat loss because the contribution of this heat loss could be simulated by increasing the surface area of the simulator. Increasing the surface area on the greater than 2500 gram simulators would not significantly alter the surface to mass ratio. However for infants with less than 2500 gram birth weights, in particular infants less than 1000, the water losses for these infants are so high in the first week of life that simulating this mode of heat loss by increasing the surface area of the simulator would significantly alter the surface to mass ratio, therefore maintaining the appropriate balance between the various compartments of heat loss (conductive, convective, radiative and evaporative) and simulating the correct surface to mass ratio would be critical to effective dynamic heat simulation. The paper proposed the use of a known formula for correlating infant surface area to the weight and height of the infant: EQU Surface Area (m.sup.2)=Weight.sup.0.5378 (kg).times.Length.sup.3.965 (cm).times.0.024265
The paper noted that while the specific heat capacity of premature infants had not been investigated, a reasonable assumption would be that of an adult or 0.86 cal/gm/.degree.C. The paper then noted that the coefficient of thermal conduction for an adult ranges from 0.47 to 0.6 W/m.degree.C. in a vasoconstricted state and from 3.76 to 4.81 W/m.degree. C. in a vasodialated state. The paper also noted that the metabolic heat production for infants was estimated to range from a minimum of 1.75 to a maximum of 4.37 kcal/kg/hr, and that the insensible water loss rate for infants less than 1,250 grams ranges from 2.4 to 4.08 g/kg/hr, translating into a range of heat loss from 1.24 to 2.37 kcal/kg/hr. With this information, the Donnelly paper then proposed the construction of a simulator utilizing copper cylinders painted with black matte lacquer paint and filled with water having a specific heat capacity of 0.99 cal/g/.degree.C. at 37.degree. C. which would simulate the specific heat capacity of human tissue. The paper also proposed simulating metabolic heat production by including an aquarium heater in one of the simulator cylinders. The Donnelly paper noted that evaporative heat and water losses were difficult to simulate, but suggested one approach might be the use of synthetic skin as the outer surface for a simulator.
These two proposed simulators were not without criticism however. Neither included any means for simulating insensible water loss nor any means for simulating a fluid circulatory system which would mimic advective heat transfer fluid flow to the limbs of the simulator. As described above, the lack of any means with which to simulate insensible water loss leads to simulation errors of up to about 50%. The lack of any means for simulating a circulatory system can lead to a "hot spot" in the simulator in the area of the heater or the circulating pump, which adversely affects the ability of the simulator to adequately model infant static and dynamic thermal responses.